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ARO Operating Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
ARO Operating Costs
Accretion	$ 1,973	$ 1,894	$ 5,990	$ 6,189
Depreciation	1,110	1,947	4,837	5,342
Total	$ 3,083	$ 3,841	$ 10,827	$ 11,531

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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- Definition

Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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